Ionic compounds with metallic conductivity

From the discussion of Chapter I, it follows that metallic conduction is to be associated with partially filled bands of collective-electron states. Since the s-p bands of an ionic compound are either full or empty, metallic conduction implies partially filled d bands, and collective d electrons imply Rtt < Rc(n,d). From the requirement Rtt < Rc(n4) it is apparent that the metallic conduction in ionic compounds must be restricted either to transition element compounds in which the anions are relatively small or to compounds with a cation/anion ratio > 1. Also Rc(n,d) decreases, for a given n, with increasing atomic number, that is with increasing nuclear charge, and the presence of eQ electrons increases the effective size of an octahedral cation (627) (see Fig. 66) and similarly UQ electrons the size of a tetrahedral cation. It follows that If the cation/anion ratio < 1, MO d electrons are more probable in ionic compounds with octahedral-site cations if the cations contain three or less d electrons MO d electrons are more probable in ionic compounds with tetrahedral-site cations if the cations contain two or less d electrons. 

Hitherto we have dealt with model FICs that are mostly useful as solid electrolytes. The other class of compounds of importance as electrode materials in solid state batteries is mixed electronic-ionic conductors (with high ionic conductivity). The conduction arises from reversible electrochemical insertion of the conducting species. In order for such a material to be useful in high-energy batteries, the extent of insertion must be large and the material must sustain repeated insertion-extraction cycles. A number of transition-metal oxide and sulphide systems have been investigated as solid electrodes (Murphy Christian, 1979). 

The hydrides, oxides and nitrides of the elements of the first three groups of the periodic system all behave as normal ionic compounds. They all show normal valencies, they crystallize in coordination lattices, do not show any metallic conductivity and, in so far as an estimate can be made of their heats of formation, they agree fairly well with the values to be expected for ionic compounds. 

A galvanic cell consists of two electrodes, or metallic conductors, that make electrical contact with the contents of the cell, and an electrolyte, an ionically conducting medium, inside the cell. The electrolyte is typically an aqueous solution of an ionic compound, although advanced cells make use of all kinds of exotic materials (see Box 12.1). 

Most of the Group IA and IIA metals react with hydrogen to form metal hydrides. For all of the metals in these two groups except Be and Mg, the hydrides are considered to be ionic or salt-like hydrides containing H ions (see Chapter 6). The hydrides of beryllium and magnesium have considerable covalent character. The molten ionic compounds conduct electricity, as do molten mixtures of the hydrides in alkali halides, and during electrolysis of the hydrides, hydrogen is liberated at the anode as a result of the oxidation of H ... 

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